The knife is the last thing you should have to think about. Keep it simple, keep it useful: a utility knife you use every day will be where your hand knows to get it and open it without fumbling.
Attitude, alertness, and training are MUCH more important. A knife, stick, pen, handful of sand ... or a pistol ... are no use if you're blindsided or DEAD before you realize you need to use them.
Self-defense means keeping yourself from getting hurt.
Fighting, unfortunately, is a classic way to get hurt.
The basic rules are:
* Don't go there.
* If you find yourself there, leave.
* If you can't leave, then fight your way out.
* None of this includes fighting for its own sake because you were insulted, or challenged, or feuding with someone.